Effects of weak acids on proline accumulation in barley leaves: a comparison between abscisic acid and isobutyric acid

Abstract. When isobutyric acid (IBA) or abscisic acid (ABA) are supplied to leaf sections a similar rapid and marked decrease in the intracellular pH is observed. This acidification is accompanied by an increase in proline level which is about the same for both 3 mol m⁻³ IBA and 1 mol m⁻³ ABA treatments.
Fusicoccin (FC), known to act at the proton pump level, almost completely suppresses the ABA-induced acidification of the cell sap, whereas it only partially counteracts the acidifying effect of IBA, in particular during short periods of treatment. This effect of FC is paralleled by a similar inhibition of the induced proline accumulation: in fact, FC completely suppresses the ABA-induced increase in proline during short treatment periods, whereas it is only effective in inhibiting the IBA-induced proline accumulation after long treatment periods.
These data seem to suggest that the ABA- and IBA-induced changes in proline level might be mediated by changes in the intracellular pH.     

Effect of boundary layer conductance on the response of stomata to humidity

Abstract. Leaf conductance responses to leaf to air water vapour partial pressure difference (VPD) have been measured at air speeds of 0.5 and 3.0 ms⁻¹ in single attached leaves of three species in order to test the hypothesis that leaf conductance response to VPD is controlled by evaporation from the outer surface of the epidermis, rather than by evaporation through stomata. Total conductance decreased linearly with increassing VPD at both air speeds, but was decreased 1.6 3.0 times as much as by a given incrase in VPD at high than at low air speed. depending on species. In all species the relationship between leaf conductance and the gradient for evaporation from the epidermis was the same at both values of boundary layer conductance, supporting the hypothesis that direct epidermal evaporation controls stomatal guard cell behaviour in responses of stomata to VPD in these species.     

Respiration-dependent proton translocation and the mechanism of protonmotive force generation in Nitrobacter winogradskyi

Abstract Proton translocation associated with electron flow to oxygen has been observed with cells of Nitrobacter winogradskyi in the presence of either potassium ferrocyanide or isoascorbate plus N , N , N ', N ' tetramethyl- p -phenylenediamine. The data are consistent with a proton pumping function for the terminal oxidase, cytochrome aa ₃, in this organism as the mechanism for generating a protonmotive force. The failure of previous work with Nitrobacter [4] to detect proton translocation linked to oxidation of nitrite, the physiological substrate, is discussed.     

In vitro metabolism of N-methyl-dibenzo [c,g]carbazole a potent sarcomatogen devoid of hepatotoxic and hepatocarcinogenic properties

The metabolism of N-methyl substituted 7H-dibenzo[c,g]carbazole (N-Me DBC) was investigated in vitro using liver microsomes from 3-methylcholanthrene (MC)-, benzo[c]carbazole (BC) and Arochlor-pretreated mice and rats. N-Me DBC is a potent sarcomatogen devoid of hepatotoxicity and liver carcinogenic activity. The ethyl acetate-extractable metabolites were separated by high performance liquid chromatography (HPLC) and most of them were identified by proton magnetic resonance (PMR), mass spectrometry (MS) and comparison with synthetically prepared specimens. Mouse and rat microsomes gave rise to the same metabolites. The major metabolites were 5-OH-N-Me DBC (50%), N-hydroxymethyl (HMe) DBC (25-30%) and 3-OH-N-Me DBC (10%). Addition of 1,1,1-trichloropropene-2,3-oxide (TCPO) to the standard incubation medium permitted the identification of two dihydrodiols among the minor metabolites. No metabolite of DBC was observed after incubation of N-Me DBC, or its major metabolite N-HMe DBC, with either mouse or rat microsomes, but the possibility of a slight demethylation cannot be totally excluded. The lack of biotransformation at the nitrogen atom site may explain the lack of hepatotoxicity and liver carcinogenic activity of N-Me DBC. The modulation of metabolism by epoxide hydrolase, cytosol and glutathione was also investigated. The results are discussed in the light of data previously obtained with hepatotoxic and hepatocarcinogenic DBC.     

Oscillations in stomatal conductance of hybrid poplar leaves in the light and dark

Cycling of stomatal conductance in three hybrid poplar ( Populus sp.) cultivars was observed under a variety of conditions. Illumination of plants kept previously in the dark induced very large oscillations with a period of about 40 min and large oscillations with a shorter period (< 10 min) were superimposed on the longer cycles. During these oscillations, large changes in conductance could occur very rapidly (1.0 cm s⁻¹ in 3 min). Plants in constant light also displayed both long and short term cycles in conductance, but these were smaller in amplitude than those induced by sudden illumination. Stomatal oscillations were also observed in darkness and after darkening of previously illuminated plants. These oscillations had shorter (< 30 min) and less regular periods than those observed in the light. Such cycling in the dark is rare. Cycling of the two leaf surfaces was sometimes in synchrony in the light, and more so after a perturbation. Little synchrony between the two surfaces was observed in the dark. Stomatal movements of different leaves on a plant were usually relatively independent. Transient stomatal opening occurred following leaf excision in the light or dark, and often after sudden darkening of intact leaves. Also, stomata of intact leaves sometimes transiently closed following illumination.     

Effects of antidiuretic hormone on cellular conductive pathways in mouse medullary thick ascending limbs of Henle: II. Determinants of the ADH-mediated increases in transepithelial voltage and in net Cl− absorption

Summary Cellular impalements were used in combination with standard transepithelial electrical measurements to evaluate some of the determinants of the spontaneous lumen-positive voltage,V _e , which attends net Cl^– absorption,J _Cl ^net , and to assess how ADH might augment bothJ _Cl ^met andV _e in the mouse medullary thick ascending limb of Henle microperfusedin vitro. Substituting luminal 5mm Ba⁺⁺ for 5mm K⁺ resulted in a tenfold increase in the apical-to-basal membrane resistance ratio,R _c /R _bl , and increasing luminal K⁺ from 5 to 50mm in the presence of luminal 10^–4 m furosemide resulted in a 53-mV depolarization of apical membrane voltage,V _a . Thus K⁺ accounted for at least 85% of apical membrane conductance. Either with or without ADH. 10^–4 m luminal furosemide reducedV _e andJ _Cl ^net to near zero values and hyperpolarized bothV _a andV _bl , the voltage across basolateral membranes; however, the depolarization ofV _bl was greater in the presence than in the absence of hormone while the hormone had no significant effect on the depolarization ofV _a , Thus ADH-dependent increases inV _b were referable to greater depolarizations ofV _bl in the presence of ADH than in the absence of ADH 68% of the furosemide-induced hyperpolarization ofV _a was referable to a decrease in the K⁺ current across apical membranes, but, at a minimum, only 19% of the hyperpolarization ofV _bl could be accounted for by a furosemide-induced reduction in basolateral membrane Cl^– current. Thus an increase in intracellular Cl^– activity may have contributed to the depolarization ofV _bl during net Cl^– absorption, and the intracellular Cl^– activity was likely greater with ADH than without hormone. Since ADH increases apical K⁺ conductance and since the chemical driving force for electroneutral Na⁺,K⁺,2Cl^– cotransport from lumen to cell may have been less in the presence of ADH than in the absence of hormone, the cardinal effects of ADH may have been to increase the functional number of both Ba⁺⁺-sensitive conductance K⁺ channels and electroneutral Na⁺,K⁺,2Cl^– cotransport units in apical plasma membranes.     

Regulation of the basolateral potassium conductance of theNecturus proximal tubule

Summary Two methods, the measurement of the response of the basolateral membrane potential (V _bl) of proximal tubule cells ofNecturus to step changes in basolateral K⁺ concentration, and cellular cable analysis, were used to assess the changes in basolateral potassium conductance (G _K) caused by a variety of maneuvers. The effects of some of these maneuvers on intracellular K⁺ activity (a _K ⁱ ) were also evaluated using double-barreled ion-selective electrodes. Perfusion with 0mm K⁺ basolateral solution for 15 min followed by 45 min of 1mm K⁺ solution resulted in a fall in basolateral potassium (apparent) transference number (t _K),V _bl anda _K ⁱ . Results of cable analysis showed that total basolateral resistance,R _b , rose. The electrophysiological effects of additional manipulations, known to inhibit net sodium reabsorption across the proximal tubular epithelium ofNecturus, were also investigated. Ouabain caused a fall int _K accompanied by large decreases ina _K ⁱ andV _bl. Lowering luminal sodium caused a fall int _K and a small reduction inV _bl. Selective reduction of peritubular sodium, a maneuver that has been shown to block sodium transport from lumen to peritubular fluid, also resulted in a significant decrease int _K. These results suggest thatG _K varies directly with rate of transport of the sodium pump, irrespective of the mechanism of change in pump turnover.Part of this material has been presented at the 10th International Conference on Biological Membranes (Cohen & Giebisch, 1984).     

Slow potential changes due to transport number effects in cells with unstirred membrane invaginations or dendrites

Summary Many neurones are extremely invaginated and possess branching processes, axons and dendrites. In general, they are surrounded by a restricted diffusion space. Many of these cells exhibit large, slow potential changes during the passage of current across their membranes. Whenever currents cross membranes separating aqueous solutions, differences in transport numbers of the major permeant ions give rise to local concentration changes of these ions adjacent to the membranes, which will result in various electrical and osmotic effects. These transport number effects are expected to be enhanced by the presence of membrane invaginations. Dendrites are equivalent to reversed invaginations and there should be significant changes in concentrations of permeant ions within them. In general, the effects of such changes on the electrical response of a cell will be greater when the concentration of a major permeant ion is low. The effects have been modelled in terms of two nondimensional parameters: the invagination transport number parameter and the relative area occupied by the invaginations A. If these two parameters are known, the magnitudes and time course of the slow potential changes can immediately be estimated and the time course converted to real time, if the length of the invaginations (l) and ionic diffusion coefficient (D) within them are also known. Both analytical and numerical solutions have been given and predictions compared. It is shown that in the case of large currents and potentials the analytical solution predictions will underestimate the magnitudes and rates of onset of the voltage responses. The relative magnitude of the transport number effect within the invaginations (or dendrites) and other transport number contributions to slow potential changes have also been assessed and order-of-magnitude values of these are estimated for some biological data.     

Membrane potential and surface potential in mitochondria: Uptake and binding of lipophilic cations

Summary The uptake and binding of the lipophilic cations ethidium⁺, tetraphenylphosphonium⁺ (TPP⁺), triphenylmethylphosphonium⁺ (TPMP⁺), and tetraphenylarsonium⁺ (TPA⁺) in rat liver mitochondria and submitochondrial particles were investigated. The effects of membrane potential, surface potentials and cation concentration on the uptake and binding were elucidated. The accumulation of these cations by mitochondria is described by an uptake and binding to the matrix face of the inner membrane in addition to the binding to the cytosolic face of the inner membrane. The apparent partition coefficients between the external medium and the cytosolic surface of the inner membrane (K' _o) and the internal matrix volume and matrix face of the inner membrane (K' _i) were determined and were utilized to estimate the membrane potential from the cation accumulation factorR _c according to the relation =RT/ZF ln [(R _cV_o–K'_o)/(V_i+K'_i)] whereV _o andV _i are the volume of the external medium and the mitochondrial matrix, respectively, andR _c is the ratio of the cation content of the mitochondria and the medium. The values of estimated from this equation are in remarkably good agreement with those estimated from the distribution of⁸⁶Rb in the presence of valinomycin. The results are discussed in relation to studies in which the membrane potential in mitochondria and bacterial cells was estimated from the distribution of lipophilic cations.